Implants intended for implantation in the human body are frequently made of thermoplastics, such as polymethylmethacrylate, polypropylene, polyethylene, and other materials which cannot be sterilized by either heating or by autoclaving. The sterilization process currently employed for thermolabile objects is that of gas absorption with ethylene oxide. To be sure, the use of ethylene oxide is associated with a high degree of risk, inasmuch as this sterilizing agent is a carcinogen, exhibits a high general potential for toxicity, and is subject to reports of intraocular irritation when used to sterilize intraocular lens implants. Ethylene oxide can thus induce serious toxic systemic effects when used to sterilize an implant, if the gas absorbed by the biomaterial of the implant and the reaction products (chlorohydrine) arising from reaction with water are not quantitatively removed from the biomaterial; due to its high toxicity, ethylene oxide also represents a serious pollutant. Despite these risks, ethylene oxide is routinely used to sterilize thermolabile pharmaceutical agents and thermolabile biomaterials, such as implants, stitching material, operating accessories, etc., since it is currently the only sure method for sterilizing these kinds of thermolabile materials.
Previously employed processes, e.g. sterilization with 10% sodium hydroxide (wet sterilization), have proven to be unreliable. In the case of sterilization of intraocular lenses of polymethylmethacrylate (PMMA), two serious epidemic intraocular infections arose in the United States in 1978, one from mushroom-contaminated sodium hydroxide and another from neutralizing solution contaminated with Pseudomonas. Due to this series of infections, the US health agency, the FDA, has permitted only the gas sterilization of intraocular lenses. There remains considerable uncertainty, however, concerning the quantity of residual ethylene oxide in the treated material. In the case of intraocular lenses the FDA permits a residual content for ethylene oxide and chlorohydrine of 25 ppm per lens for either substance. The French health authority sets the maximum residual content for ethylene oxide at 2 ppm per lens. The German Federal Office of Health recommends a limiting value of 1 ppm for ethylene oxide and 150 ppm for chlorated ethylene hydrine as applied to intraocular lenses.
In the alternative use of radio-sterilization employing gamma radiation, the danger arises of a structural modification of the biomaterial, thereby raising questions about the suitability of the radiated implant for the intended medical purpose. Changes occurring in the material call in question the suitability of the implant, particularly when the treated biomaterial is completely penetrated by the gamma rays during treatment. For example, in the case of intraocular lenses, the lenses have relatively thin attachment threads by means of which they are attached to the body at the site of implantation. Gamma radiation creates the danger of rendering these attachment threads brittle, so that they will no longer be able to fulfill their function. In addition, gamma radiation results in darkening and discoloration of the lens material. Due to the high risks associated with radio-sterilization employing gamma rays, high requirements must be met with respect to qualitative change, radiolysis products, storage, and the like. Consequently the conditions demanded for the licensing of radio-sterilization procedures cannot presently be fulfilled for medically employed biomaterial, and the substitution of gamma radiation for ethylene oxide is not currently possible.